Test io_uring flags before using them

is there a better way?

https://github.com/axboe/liburing/issues/906

src/fellow_io_uring.c

@@ -226,6 +226,42 @@ test_task(struct worker *wrk, void *priv)
 /* XXX very simplistic. Sufficient for fellow, but not otherwise */
 static int shared_wq_fd = -1;
 
+/* XXX BETTER WAY? https://github.com/axboe/liburing/issues/906 */
+static unsigned flags_checked = 0;
+static unsigned flags_supported = 0;
+
+static unsigned
+try_flag(unsigned flag)
+{
+	struct io_uring_params params;
+	struct io_uring ring;
+	int ret;
+
+	memset(&params, 0, sizeof params);
+	params.flags = flag;
+
+	ret = io_uring_queue_init_params(2, &ring, &params);
+	if (ret == 0) {
+		io_uring_queue_exit(&ring);
+		return (flag);
+	}
+
+	assert(ret == -EINVAL);
+	return (0);
+}
+
+static void
+try_flags(void)
+{
+
+	if (flags_checked != 0)
+		return;
+#ifdef IORING_SETUP_COOP_TASKRUN
+	flags_supported |= try_flag(IORING_SETUP_COOP_TASKRUN);
+#endif
+	flags_checked = 1;
+}
+
 fellow_ioctx_t *
 fellow_io_init(int fd, unsigned entries, void *base, size_t len,
     fellow_task_run_t taskrun)
@@ -236,6 +272,8 @@ fellow_io_init(int fd, unsigned entries, void *base, size_t len,
 	int ret, answer = 0;
 	fellow_task_privstate taskstate;
 
+	try_flags();
+
 	probe = io_uring_get_probe();
 	if (probe == NULL) {
 		fprintf(stderr, "io_uring_get_probe() failed\n");
@@ -250,10 +288,7 @@ fellow_io_init(int fd, unsigned entries, void *base, size_t len,
 	AZ(taskrun(test_task, &answer, &taskstate));
 
 	memset(&params, 0, sizeof params);
-
-#ifdef IORING_SETUP_COOP_TASKRUN
-	params.flags |= IORING_SETUP_COOP_TASKRUN;
-#endif
+	params.flags = flags_supported;
 
 #ifdef IORING_SETUP_ATTACH_WQ
 	if (shared_wq_fd >= 0) {